1. Field of the Invention
The present invention relates to improved apparatus and methods for operating fluorescent lamps and, in particular, to a method and apparatus to control the power delivered to a fluorescent lamp.
2. Description of the Prior Art
Fluorescent lamps are conventional types of lighting devices. They are gas charged devices which provide illumination as a result of atomic excitation of a low-pressure gas, such as mercury, within a lamp envelope. The excitation of the mercury vapor atoms is provided by a pair of heater filament elements mounted within the lamp at opposite ends of the lamp envelope. In order to properly excite the mercury vapor atoms, the lamp is ignited or struck by a higher than normal voltage. Upon ignition of the lamp, the impedance decreases and the voltage across the lamp drops to the operating level at a relatively constant current. The excited mercury vapor atoms emit invisible ultraviolet radiation which in turn excites a fluorescent material, e.g., phosphor, that is deposited on an inside surface of the fluorescent lamp envelope, thus converting the invisible ultraviolet radiation to visible light. The fluorescent coating material is selected to emit visible radiation over a wide spectrum of colors and intensities.
As is known to those skilled in the art, a ballast circuit is commonly disposed in electrical communication with the lamp to provide the elevated voltage levels and the constant current required for fluorescent illumination. Typical ballast circuits electrically connect the fluorescent lamp to line alternating current and convert this alternating current provided by the power transmission lines to the constant current and voltage levels required by the lamp.
Fluorescent lamps have substantial advantages over conventional incandescent lamps. In particular, the fluorescent lamps are substantially more efficient and typically use 80 to 90% less electrical power than incandescent lamps for an equivalent light output. For this reason, fluorescent lamps have gained use in a wide range of power sensitive applications.
To strike, or light, a fluorescent lamp a high voltage level is required rather than high power. To achieve this high voltage, many current fluorescent lamp ballast designs dissipate high power levels
Additionally, at the end of a lamps life-cycle, the voltage required to strike the lamp increases due to the depletion of the electron emitting coating on the filament heater element. This results in the lamp not lighting because the peak voltage required to strike the lamp cannot be provided by conventional ballasts. Also, low temperatures require a higher voltage to strike the lamp. This is due to the greater thermal gradient between the environmental temperature and that necessary to ignite the lamp. This limits the environmental operating range of current ballast designs, making them unusable at low wintertime temperatures.
In addition present ballasts are limited in their ability to accommodate a range of tube lengths. This being the case, the circuit is normally tuned to the specific lamp load. Many designs require that when lamps are hot changed, i.e., circuit power is not turned off, that the circuit be reset.